The aim of seismic exploration is to determine and display the nature of the subsurface structure of the earth by seismic methods. Exploration seismology is divided into two branches: reflection seismology and refraction seismology. Reflection seismology is a method of mapping the subsurface sedementary rock layers from measurements of the arrival times of events reflected from the subsurface layers. The present invention is applicable primarily to reflection seismology.
The seismic field in the Earth is generated by a source, or by plurality of sources, located on or near the Earth's (so-called) free surface. The seismic waves propagate down and reflect from subsurface interfaces. The reflected waves return to the free surface and are detected by a plurality of receivers located along a seismic line or at grid points (FIG. 1). These receivers transform the seismic oscillations into electrical signals which are suitable for later processing. This technique is called "reflecting shooting".
The main object of data processing in reflecting shooting is to select useful reflected waves (primaries) carrying information about deep sub-surface structures, and to suppress unwanted signals (multiples, refracted, surface and other types of waves) and noise.
One of the most powerful tools in reflected waves data processing is the so-called Common Depth Point (CDP) method. The seismic data in the CDP method is generated and collected by source-receiver pairs via ray paths shown in FIG. 2. The midpoint between source and receivers is labeled Ao. The positions of a source and a receiver corresponding to the one trace (pair) are labeled Ak.sup.+ and Ak.sup.-. Their coordinates X.sup.+.sub.k and X.sup.-.sub.k are determined by the relationship: ##EQU1## Thus all pairs Ak.sup.+ and Ak.sup.- have the same middle point; therefore the CDP method is also called the Common Middle Point (CMP) method.
The display of seismic traces associated with the CDP configuration of source-receiver pairs is called the CDP (or CMP) gather, or the CDP seismogram. Summing (or stacking) of traces of a CDP gather is based to the CDP method.
It is necessary to introduce so called static and dynamic correction to each trace before stacking. The static correction is to compensate for the irregularities in reflection arrival times resulting from near surface variation in elevation, weathering thickness, and weathering velocity. The object of the static correction is to reduce each trace to a "flat" reference (datum) level without the weathering zone.
The dynamic correction is a time-device correction which is needed because of the existence of a difference .DELTA..tau.k of reflection arrival times at the (k-th) trace and of the predetermined central (zero offset) trace t.sub.o. The value .DELTA..tau.k is called a Normal Moveout (NMO) and can be computed with the help of the following approximate relationship: ##EQU2## where Vs(to) is a so-called stacking velocity for zero time to. If values of Vs are correctly selected, the NMO correction puts all primary reflections on traces of a CDP gather in phase; after this, all traces of a CDP gather are stacked. Thus, one output trace, called a stacked trace, is obtained for each middle point Ao. In many cases primaries on an optimal stacked trace are enhanced against unwanted waves and noise.
A stacked section, called a time section can be obtained by repeating the stack for a sequence of middle points and plotting the obtained CDP stacked traces .mu.(xo,to) as a two-dimentional function of middle point coordinate xo and the half two-way zero time to. The time section is an image of a subsurface structure, but it is a distorted image because reflections from dipping and curved interfaces actually come not from directly under the halfway position as plotted. To remove this distortion a special, technique called migration, is applied to time sections.
Usually summation of fields causes a decrease of resolution. However, if the reflectors are horizontal, or near-horizontal, and the overburden is a horizontal stratified medium, then the CDP stack is not accompanied by a noticeable diminution of resolution as compared with unstacked traces, because all traces of the CDP gather correspond to one specular point (FIG. 2). If the reflector is a dipping interface, and/or the overburden is not a horizontal layered medium, the different specular points on a reflecting surface correspond to different traces Uk(.DELTA. X) of the CMP gather (FIG. 3). In these cases, stacking of traces is accompanied with a decrease in resolution (smearing of the reflecting surface). This effect is especially present for reflectors with complex topography (FIG. 3a) or with noticeable variation of reflecting properties along the surface.
To avoid such a smearing of the reflector and other accompanying unwanted effects, special methods of processing multifold data, called Partial Prestack Migration (PPM) or Dip Moveout (DMO), have been developed. There are different modifications of this method. In all these modifications, in order to improve the resolution of the stacking process, it is necessary that reflected events on all field traces in the stacked gather correspond to a Common Reflecting Point (CRP) as defined by the normal incident ray (FIG. 4). To achieve this goal, the DMO data processing includes both a space correction and a time correction for each sample of an input field datum. After migrating the prestack data to the true position, the common midpoint stack is performed. In order to actually find the true values of the space and time corrections, it is necessary in the DMO method to have initially a good estimate model of the overburden and the dipping of the reflector.